On 1/3/2011 12:27 PM, jimlux wrote:
> Mark Robinson wrote:
>>> NEC doesn't say anything about multiple rods, other than they have to be
>>> bonded together. (and, of course, that a rod can't be the sole
>>> grounding means in a new installation)
>> What do they require now? Several rods. I just beefed up my service
>> ground (an old steel pipe banged in the ground 30 years ago) with
>> another 10 foot copper clad rod right by it and then tied the system
>> into the 30 ground rods in my tower grounding system.
>>
> Here's what Art 250.50 of 2005 NEC says:
> 1) metal underground water pipe at least 10 feet long that is outside
> the building. A well casing will do.
Although it's in the NEC a well casing can be a very bad idea and it'd
never make code here. one, you can't have the well that close and two,
the ground rods must be within about 6 or 8 feet of the service
entrance. Getting both requirements is almost impossible. I had to put
in two ground rods to satisfy the local code when we went to the 200 amp
service.
I say the well casing is a bad idea as many wells have submersible pumps
which are easily damaged by lightning strikes the local electricians
tell me and it does sound plausible.
All water pipe around here is plastic. However code requires that you
tie you ground to the water meter (which is metal with plastic pipe
going in and coming out) The gas line must also be tied into the ground
system. I really never liked that idea.
73
Roger (K8RI)
> 2) metal frame of building, as long as at least 10 feet of a single
> structural member is in contact with the earth or encased in concrete
> which is in contact with the earth. (plus some other restrictions)
> 3) A Concrete encased grounding electrode (Ufer ground) 20 ft of AWG 4
> bare copper encased in at least 2" of concrete in contact with earth.
> Rebar also works.
> 4) A ground ring of AWG 2 bare copper at least 20 ft long.
> 5) Rod and pipe.. at least 8 feet in contact with soil,5/8" diameter
Hasn't that in some cases been increased to 10' of 3/4"?
> 6) A plate of at least 2 square feet of surface area, no thinner than 1/4"
>
>
> As far as spacing goes:
> 250.53(B) says that electrodes of types 5 and 6 (rods and plates) must
> be spaced at least 6 feet apart and from other grounding electrodes
> (e.g. for air terminal grounding).
>
> if you want to use #1, 250.53(D)(2) says you have to have another
> electrode as well
>
> 250.56 says if you're using rods/plates, the resistance has to be<25
> ohms and multiple rods have to be at least 6 ft apart.
>
> 250.53(G) says that if you bury the rod horizontally (say, you're on a
> rock ledge), it has to be at least 30" deep.
>
> Multiple rods can be used if measured resistance is<25 ohms, but
> there's a bunch of other rules too. (8 feet of rod in contact with soil,
> etc.)
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